The president of Toyota's U.S. operations acknowledged to skeptical lawmakers on Tuesday that the company's recalls of millions of its cars may "not totally" solve the problem of sudden and dangerous acceleration.

"We are vigilant and we continue to look for potential causes," Toyota's James Lentz told a congressional panel. However, he repeated his company's position that unexpected acceleration in some of the company's most popular cars and trucks was caused by one of two problems - misplaced floor mats and sticking accelerator pedals.

He insisted electronic systems connected to the gas pedal and fuel line did not contribute to the problem, drawing sharp criticism from lawmakers who said such a possibility should be further explored - and from a tearful woman driver who could not stop her runaway Lexus.

"Shame on you, Toyota," Rhonda Smith, of Sevierville, Tenn., said at a congressional hearing. Then she added a second "shame on you" directed at federal highway safety regulators.

Texas Republican Rep. Joe Barton cautioned his colleagues early in the hearing against conducting a "witch hunt" and said "We don't want to just assume automatically that Toyota has done something wrong and has tried to cover it up." But midway through Lentz's testimony, Barton said of Toyota's investigation of the problems: "In my opinion, it's a sham."

Lentz said the company had not completely ruled out an electronics malfunction and was still investigating causes of the sudden acceleration. Still, "We have not found a malfunction" in the electronics of any of the cars at issue, he said.

As to Smith's harrowing story, "I'm embarrassed for what happened," Lentz said. "I want her and her husband to feel safe about driving our products," Lentz said.

At one point in more than two hours of testimony Lentz was asked by Rep. Eliot Engel, D-N.Y., whether there were any new bombshells to come.

"God, I hope there aren't any more," he said, while apologizing anew for the problems. "We stubbed our toe," he said.

Three congressional panels are investigating Toyota's problems, which affect a huge number of Americans. Toyota has recalled some 8.5 million vehicles worldwide - more than 6 million in the United States - since last fall because of unintended acceleration problems in multiple models and braking issues in the Prius hybrid. It is also investigating steering concerns in Corollas. People with Toyotas have complained of their vehicles speeding out of control despite efforts to slow down, sometimes resulting in deadly crashes. The government has received complaints of 34 deaths linked to sudden acceleration of Toyota vehicles since 2000.

Lentz, who choked up while discussing the death of his own brother more than 20 years ago in a car accident, said he understood the pain.

"I know what those families go through," he said.

Lentz has said in the past that he was confident Toyota's fixes on the recalled vehicles would correct the problems.

But when pressed by Energy and Commerce Committee Chairman Henry Waxman, D-Calif., on whether the two recalls Toyota put in place to deal with the issue would completely solve it, Lentz replied: "Not totally."

Still, he said chances of unintended accelerations were "very, very slim" once the recall was complete. Lentz also said Toyota was putting in new brakes that can override the gas pedal on almost all of its new vehicles and a majority of its vehicles already on the road.

Meanwhile, Toyota president Akio Toyoda, who will testify before a separate panel on Wednesday, said he took "full responsibility" for the uncertainty felt by Toyota owners and offered his condolences to a San Diego, Calif., family who were killed in late August, reigniting interest in the problems.

"I will do everything in my power to ensure that such a tragedy never happens again," Toyoda said in prepared testimony for Wednesday's hearing to the House Government Oversight Committee. "My name is on every car. You have my personal commitment that Toyota will work vigorously and unceasingly to restore the trust of our customers."

Lawmakers heard a brief, but riveting, description from Smith, the Tennessee woman whose Toyota-made Lexus suddenly zoomed to 100 miles per hour as she tried to get it to stop - shifting to neutral, trying to throw the car into reverse and hitting the emergency brake. Finally, her car slowed enough that she was able to pull it off the road onto the median and turn off the engine.

Fighting back tears, she described her nightmare ride of October 2006, calling it "a near death experience."

"After six miles, God intervened" and slowed the car, she said. She added that it took a long time for Toyota to respond to her complaints.

In an often contentious full day of testimony, lawmakers returned again and again to the question of whether electronic malfunctions may have contributed to the speeding cars.

"We are confident that no problems exist with the electric throttle control system in our vehicles," Lentz said. He cited "fail-safe mechanisms" in the cars that were designed to shut off or reduce engine power "in the event of a system failure."

Transportation Secretary Ray LaHood told the panel that possible electronics problems were being looked into by his agency. He said the company's recalls were important steps but "we don't maintain that they answer every question."

Toyota hired a consulting firm to analyze whether electronic problems could cause unintended acceleration. The firm, Exponent Inc., found no link between the two. But committee investigators said the testing studied only a small number of vehicles

Tracking down an electrical problem can be far more difficult, expensive and time-consuming than finding a mechanical problem. Electrical problems can have more than one source, and they can come from inside or outside the car. Mechanical problems often leave clues such as physical damage, where electronic troubles can be hidden in software or leave no trace at all.

House investigators who reviewed Toyota's customer call database found that 70 percent of the complaints of sudden acceleration were for vehicles that are not subject to the recalls over floor mats or sticky pedals.

Lentz is president and chief operating officer of Toyota Motor Sales USA Inc.

Separately, among hundreds of Toyota dealers lobbying members of Congress Tuesday, there seemed to be widespread rancor toward a federal government they view as picking on the automaker, at least in part because of the government's investment of billions of dollars in General Motors and Chrysler.

"That's hard for me as a citizen to understand why my tax dollars are going in that direction," Paul Atkinson, a Houston-area Toyota dealer, said at a news conference that also served as a pep rally for the visiting dealers. "To compete with the government as an individual entrepreneur is pretty tough."